The presence of localized nonuniformities in luminescing materials, including semiconductors, is of both theoretical and practical importance. The practical importance of understanding and analyzing such nonuniformities, for example, defects, dislocations and growth striations in semiconductors arises because they may greatly alter device performance even to the point of rendering the device inoperative or greatly reducing device efficiency and lifetime. Early detection of such nonuniformities in the semiconductor device processing sequence is desirable because of the potential savings in device processing costs as the device processing sequence may be terminated if the semiconductor crystal is found to be defective. Accordingly, techniques have been devised to detect and analyze nonuniformities.
Some techniques, for example, etching, are useful in studying nonuniformities such as dislocations. However, they are inherently destructive of the material under study and are therefore not useful in testing all devices going through a processing sequence.
Several nondestructive approaches to analyzing localized nonuniformities, including the use of optical and electron beam techniques, have been tried. The optical techniques are limited in resolution by the relatively long wavelength of the light used. The attention of those people studying nonuniformities in semiconductors has therefore been directed towards use of an electron beam, with its promise of resolution better than that attainable with optical techniques, to study nonuniformities. Two exemplary electron beam methdods that are representative of the prior art will be briefly described.
The first method uses an electron beam scanned across the surface of a semiconductor crystal to produce electron-hole pairs in the semiconductor. Electrical contacts are made to opposite surfaces of the semiconductor and an internal electric field will, under appropriate conditions, separate the electrons and holes and cause an external electrical current to flow. The magnitude of the current, which is a function of the electron-hole recombination time, yields information as to the presence of defects and growth striations. Nonuniformities, such as defects and growth striations, alter the recombination time and are recognized by changes in the external current. This method is commonly referred to by the acronym, EBIC, for electron beam induced current.
The second method, cathodoluminescence (CL), also scans an electron beam across the surface of the material to study localized nonuniformities. In this method, the electron beam creates electron-hole pairs which recombine and emit light which is observed. Variations in the intensity of the emitted light, which is again a function of the electron-hole recombination time, yeilds information as to the presence of surface defects and growth striations.
Although both methods permit nondestructive analysis, each has drawbacks. EBIC requires electrical contacts to be made to the semiconductor and can detect only electrically active defects. Both EBIC and cathodoluminescence permit analysis of only a surface layer having a depth equal to the range of the electron beam. Additionally, cathodoluminescence suffers from signal to noise problems and requires sophisticated collection optics that do not interfere with the electron beam.